An electronic device is comprised of electronic parts, such as laminates, printed wiring boards, and multilayer wiring boards. In the electronic parts, generally, a resin composition is used as a prepreg, a spacer, a sealer, an adhesive sheet, or the like, and the resin composition is required to have various performance or properties. For example, recently, as the capacity and density of power elements mounted on the electronic device are increasing, the resin composition and parts using the resin composition must have even more excellent heat radiating properties and water resistance than those of conventional ones.
On the other hand, high-performance plastics used in industrial applications, e.g., electric devices, electronic devices, automobiles, and OA machines are called engineering plastics and generally have a heat resistance of 100° C. or higher and excellent mechanical properties.
Conventionally, metals, typically die-casted aluminum and die-casted zinc have been used in the applications of optical parts, electric and electronic parts, and automobile parts. However, processing such metals into predetermined shapes costs much and therefore, recently, thermoplastic resins including the above engineering plastics are being used as substitutes for the metals. The resins used in these applications are required to achieve rigidity and dimensional stability as high as those of metals, therefore a molding material is used in the form of a resin composition containing a large amount of fibrous, plate-form, or particulate inorganic filler in the resin for improving the above properties. Such a resin composition has to be improved in especially heat radiating properties in addition to the rigidity and dimensional stability.
As filler for use in a semiconductor sealing resin composition, silicon dioxide (hereinafter, referred to as “silica”) and aluminum oxide (hereinafter, referred to as “alumina”) have conventionally been used. Silica has, however, poor thermal conduction properties, and a resin composition using silica as filler does not have satisfactory heat radiation to deal with the increased heat generation due to the increase of integration degree, power, or speed of a semiconductor device, causing problems in the stable operation of semiconductor. On the other hand, a resin composition using as filler alumina having higher thermal conduction properties than those of silica has improved heat radiating properties, but the alumina has high hardness and causes a problem of the production process in that a kneading machine, a molding machine, and a mold markedly wear.
For solving the problems, magnesium oxide (MgO) having a thermal conductivity higher than that of silica by single digit and equivalent to that of alumina is studied as a resin filler material for semiconductor sealing. However, MgO powder has large moisture absorption, as compared to silica powder, and therefore, when the MgO powder is used as resin filler for semiconductor sealing, absorbed water and MgO undergo hydration to cause volume expansion of the filler, leading to problems in that cracks are caused in the resin composition and that the thermal conduction properties deteriorate. Thus, for securing a long-term stable operation of semiconductor, it is an important task to impart a water resistance to the MgO powder for use in resin filler for semiconductor sealing.
For achieving the task, there has been proposed MgO powder having high acid resistance and high water resistance obtained by, for example, subjecting MgO powder to surface treatment with an acid phosphate having a C4-C30 alkyl group or a C4-C30 alkenyl group, such as C17 stearyl acid phosphate, to form a coating film of an acid phosphate compound (Patent document 1).
However, the surface treatment with an acid phosphate improves the water resistance to an extent such that water repellency is imparted to the MgO powder due to, e.g., a C17 stearyl group, and the water resistance of the MgO powder itself is improved to some extent, but the improvement is not satisfactory.
As another method of improving the water resistance of the MgO powder, there has been proposed a method for producing coated MgO powder, which comprises mixing an aluminum (Al) salt or a silicon (Si) compound with MgO powder, and removing solids from the resultant mixture by filtration, and drying and calcining it to coat the surface of the MgO powder with a coating layer comprised of a double oxide of Al or Si and Mg (see Patent documents 2 and 3).
The above method is not merely a surface treatment but coating with a double oxide obtained by reacting Al or Si on the surface of the MgO powder, and therefore the resultant coated MgO powder is considerably improved in the water resistance.
However, it is difficult to completely coat the surface with the double oxide, and a region in which the double oxide does not completely cover remains on the surface of the MgO powder and a hydration reaction proceeds in that region, thus making it difficult to meet the strict requirements of water resistance demanded in recent years.    [Patent document 1] Japanese Unexamined Patent Publication No. 2001-115057    [Patent document 2] Japanese Unexamined Patent Publication No. 2003-34522    [Patent document 3] Japanese Unexamined Patent Publication No. 2003-34523